The invention relates to the manufacture of composite articles and, more particularly, to methods for heating composite material to the required temperature in order to maintain adhesion between a composite tape and a substrate on which the tape is placed regardless of the complexity of the tape path curvature or the contour of the substrate.
Composite structures made from fiber-reinforced polymer matrix (resin) materials are commonly manufactured by progressively building up the structure with a plurality of layers of thin composite tape or tow, hereafter collectively referred to as tape, laid one layer upon another. Typically, the operation begins by laying a tape onto a starting template having a configuration generally corresponding to the desired shape of the article to be produced. A tape placement head guides a continuous tape onto the template by providing relative movement between the template and the head, such that the head moves over the surface of the template. The head usually makes repeated passes over the template in a defined pattern until the template is entirely covered. Multiple layers of tape are built up by continued passes of the head over the surface. A compaction roller is usually used for pressing the tape against the template or prior-laid layers of tape to facilitate adhesion of the tape thereto, and the tape and/or the substrate onto which it is laid are heated just prior to the tape being compacted to soften the resin and promote adhesion of the tape to the substrate.
The most commonly used heating method for heating the tape and/or substrate is to impinge the materials with a heated gas. A drawback of this approach is that the flow of gas cannot be controlled with any great precision. Consequently, the heating of the tape and substrate cannot be accurately controlled, and hence the adhesion of the tape to the substrate cannot be accurately controlled.
The present invention seeks to improve the accuracy of heating of tape and substrate materials during automated article manufacturing. More particularly, the invention employs one or more laser diode arrays for heating the tape and substrate. The laser diode array generates a field of light energy comprised of a plurality of light beams generated by a plurality of laser diodes arranged in a rectangular or two-dimensional array. The array preferably is formed by a plurality of laser diode bars stacked one atop another, each bar having a plurality of laser diodes arranged side-by-side in a widthwise direction of the bar. Each bar is configured with a lens or plurality of lenses to collimate the light emitted by each diode so as to generate a plurality of parallel light beams lying generally in a plane. The invention enables the intensity distribution of the light energy to be tailored to the particular configuration of the composite article being produced, thereby optimizing the temperature profile on the article.
In one preferred embodiment of the invention, a method for forming a composite article includes the steps of guiding the composite tape onto a surface of a substrate and pressing the tape against the substrate in a compaction region such that the tape conforms to the contour of the surface of the substrate and is adhered thereto; and irradiating an area defined by opposing surfaces of the tape and of the substrate proximate the compaction region with a field of light energy generated by a laser diode array to heat the tape and substrate, and controlling the laser diode array to independently control heating of one portion of the irradiated area relative to another portion of the irradiated area.
The laser diode array in one embodiment of the invention is positioned to direct the field of light energy onto the tape and substrate such that the widthwise direction of each bar is generally parallel to the widthwise direction of the tape. In the widthwise direction of the tape, the intensity profile of the field of light energy can be varied by controlling the current supplied to individual diodes, or to groups of diodes, within each bar independently of the other diodes or groups of diodes in the bar. In a lengthwise direction of the tape, the intensity profile of the field of light energy can be varied by controlling the current supplied to each bar independently of the other bars in the array. The invention thus enables a temperature profile over the region of the tape and substrate to be controlled in any desired manner.
Alternatively, the laser diode array can be oriented such that the widthwise direction of each bar is parallel with the lengthwise direction of the tape. In this case, the profile of the temperature across the width of the tape is controlled by controlling the light intensity of each bar independently of the other bars, and the profile of the temperature in the lengthwise direction of the tape is controlled by controlling each diode, or each group of diodes, in each bar independently of the other diodes or groups of diodes in the bar.
In another embodiment of the invention, the profile of the laser intensity across the width of the tape is varied as a function of the curvature of the path along which the tape is steered during placement. When steering a tape along a curved path at the compaction region, the material of the tape on the inner radius of the steered path must conform to a greater degree than the material on the outer radius of the path. In conventional forced-air heating of tape and substrate, there is constant uniform heating of the tape across its width, and the tape placement head typically must be slowed down in order to allow the tape to conform to the substrate without substantial puckering or wrinkling. In the present invention, the laser diode array can be controlled to produce greater heating of the tape at the inner radius of the steered path so that the material can more readily flow and conform, and lesser heating at the outer radius of the steered path where less flow is required. Additionally, when the tape placement head negotiates surfaces of concave or convex curvature, the angle of the head with respect to the surface can vary and the speed of the head can vary. For optimum heating of the tape and substrate, the heat addition should be responsive to such changes. This responsiveness cannot be achieved using conventional forced-air heating. With the present invention, however, the heating profile can be tailored to the contour of the steered path so that heating of the tape and substrate can be more nearly optimum at all times.
In a still further embodiment of the invention, the laser diode array is made up of a plurality of independently addressable zones that are positioned side-by-side along the widthwise direction of the tape. Where the tape has a width less than the width of the light field produced by the full laser diode array, the array is controlled such that less than all of the zones are powered, thereby producing a light field whose width at the tape generally matches the band width of the tape. Thus, the laser diode array can be sized such that when all zones are powered it produces a light field that is as wide as the widest tape to be placed, yet when narrower tapes are placed, the light field""s width can be reduced by powering only some of the zones. By this method, heat is applied only to areas of the substrate for which heating is required. The independently addressable zones can be made up of laser diode bars or groups of adjacent bars where the bars have their widthwise directions oriented parallel with the lengthwise direction of the tape. Alternatively, the zones can be made up of individual diodes or groups of adjacent diodes in each bar where the bars have their widthwise directions oriented parallel with the widthwise direction of the tape.
The invention also allows the heating of tapes simultaneously fed at differing feed rates to be adjusted to compensate for the different feed rates. Thus, a tape fed at a faster rate can be heated to a greater extent than another tape fed at a lower feed rate. This can be accomplished, for example, by irradiating one tape with one portion of the laser diode array and irradiating the other tape with another portion of the array, and independently controlling the two portions of the array to produce differential light intensities.
The invention also provides a method for heating tape and substrate materials enabling a differential amount of heating between the tape and substrate by controlling the intensity of light emitted by the diodes in the array so as to deliver a different amount of heating energy to the tape relative to that delivered to the substrate. The method can compensate for different rates of heat loss between the tape and substrate so that optimum heating of each can be obtained.
As noted, the invention preferably employs an array of laser diodes formed as a stack of laser diode bars. Each laser diode bar has a plurality of zones each capable of emitting a separate laser beam or a group of side-by-side laser beams. The zones of a bar are arranged in a linear or one-dimensional array, and each zone is independently addressable such that a controller connected with the array can independently control one zone relative to the other zones. An array is formed by stacking several bars one atop another, thus forming a two-dimensional array. The width of the field of light energy can be increased by employing two or more stacks located side-by-side. The intensity of light energy produced by the array can be varied along both directions of the array. The apparatus may further include a lens, mirror, fiber optic element, or other system for guiding and/or focusing the field of light energy onto the surface.
An apparatus for practicing the methods of the invention advantageously comprises a tape placement head, a compaction device, a laser diode array oriented to direct a field of light energy onto opposing surfaces of the tape and substrate, and a controller for controlling the laser diode array. The tape placement head is operable to guide at least one composite tape onto the substrate while relative movement is provided between the tape placement head and the substrate. The compaction device engages the tape in a compaction region formed between the compaction device and the substrate and presses the tape against the substrate such that the opposing surfaces thereof are placed in contact with each other. In some cases the tape material may comprise a plurality of individual bands of slit tape, or tows, that are passed through the compaction region in side-by-side or collimated orientation relative to one another.
Preferably, the tape placement head includes a frame and the compaction device comprises a roller rotatably journalled in the frame. The tape placement head guides a tape, or a plurality of collimated tapes, over the roller and the roller presses the tape(s) onto the substrate. Advantageously, the laser diode array is mounted on the frame of the tape placement head. The tape placement head can be held stationary while the substrate is moved, or the substrate can be held stationary while the tape placement head is moved for laying the tape(s) onto the substrate.
The apparatus can also include a temperature sensor for measuring the temperature of the tape material and/or substrate proximate the compaction region where the tape material is pressed onto the substrate. For example, an infrared temperature sensor can be mounted on the frame of the tape placement head for this purpose. More than one temperature sensor can be used for sensing the temperature in more than one region, or a single thermal imaging device capable of monitoring the temperature distribution in the proximity of the compaction region can be used. The temperature(s) measured by the sensor(s) can be used by the controller for regulating current supplied to the laser diodes in the array in order to maintain a desired heat and, therefore, temperature distribution on the tape and substrate.
Another way in which different rates of heating can be applied to different areas of the tape and/or substrate is to configure the laser diode array such that some diode groups produce laser light of a different wavelength from other diode groups. The rate at which composite material absorbs light energy is dependent on, among other factors, the wavelength of the incident light. Thus, a laser diode array can be formed by stacking a plurality of laser diode bars as described above, wherein some of the diode bars produce light of a different wavelength from other bars. Bars producing light of one wavelength can be used for irradiating the tape material, and bars of another wavelength can be used for irradiating the substrate. Alternatively, bars producing light of one wavelength can be used for irradiating one widthwise portion of the tape material and/or substrate, and bars of another wavelength can be used for irradiating another widthwise portion of the tape material and/or substrate. If desired, the array can also be divided into independently powered diode groups as previously described.